Modified Zagreb M 2 Index Comparison with the Randi} Connectivity Index for Benzenoid Systems

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1 CROATICA CHEMICA ACTA CCACAA 7 (2) (2003) ISSN-00-3 CCA-2870 Note Modified Zagreb M 2 Index Comparison with the Randi} Connectivity Index for Benzenoid Systems Damir Vuki~evi} a, * and Nenad Trinajsti} b a Department of Mathematics, University of Split, HR-2000 Split, Croatia b The Rugjer Bo{kovi} Institute, P. O. Box 80, HR-0002 Zagreb, Croatia RECEIVED OCTOBER, 2002; REVISED DECEMBER, 2002; ACCEPTED JANUARY 7, 2003 Key words benzenoid systems modified Zagreb M 2 index Randi} connectivity index Two theorems have been derived for benzenoid systems with the following implications: (i) for any arbitrarily large m (e.g., m = ) there is a number n and a set with m benzenoid systems with n vertices so that every pair of them posseses the same Randi} connectivity index, but no two graphs exist with the same modified Zagreb M 2 index, and (ii) for any arbitrarily large m (e.g., m = ) there is a number n and a set with m benzenoid systems with n vertices so that (a) every pair of graphs posseses a different Randi} connectivity index, (b) every pair of graphs has a different modified Zagreb M 2 index, and (c) Randi} connectivity indices order these graphs in the reverse order with respect to modified Zagreb M 2 indices. We have shown with these two theorems that the modified Zagreb M 2 index differs fundamentally from the Randi} connectivity index. INTRODUCTION The modified Zagreb M 2 index has been proposed recently. The original Zagreb M 2 index, together with the related M index, was introduced in 972 (Ref. 2) and elaborated in In recent years, mathematical properties of Zagreb indices have also been studied.,5 Both of these indices have been continuously used in QSPR and QSAR. 8 They are also included in a number of programs for the routine computation of molecular descriptors, such as POLLY, 9 DRAGON, 0 TAM, etc. The Randi} connectivity index was introduced in 975 (Ref. 2) and it soon became the most used topological index in all kinds of structure-property-activity studies. 8 The modified Zagreb M 2 index was introduced to amend the feature that the original Zagreb M 2 index gives greater weights to inner bonds and smaller weights to outer bonds. This opposes the chemists intuition that outer bonds should have greater weights than inner bonds because outer bonds are associated with a larger part of the molecular surface and are consequently expected to make a greater contribution to physical, chemical and biological properties. The Randi} connectivity index complies with chemical intuition. Here, we will show that the discriminatory power of the modified Zagreb M 2 index surpasses that of the Randi} connectivity index for certain arbitrarily large classes of benzenoid systems. * Author to whom correspondence should be addressed. ( vukicevi@pmfst.hr)

2 8 D. VUKI^EVI] AND N. TRINAJSTI] ORIGINAL AND MODIFIED ZAGREB M 2 INDICES AND THE RANDI] CONNECTIVITY INDEX The original Zagreb M 2 of a graph G is given by: 2 M 2 (G) = didj () ( ). () The modified version of () is defined as: *M 2 (G) = [ didj () ( )]. (2) The Randi} connectivity index of G is similar to the Zagreb indices: 2 (G) = [ didj () ( )] 2 /. (3) MODIFIED M 2 INDEX AND THE RANDI] CONNECITIVITY INDEX OF BENZENOID SYSTEMS Let a,b,c be natural numbers so that a b + 2 and c 2. Denote by S(a,b,c) a benzenoid system 3 so that there are 2c rows, each odd row consisting of a hexagons and each even row consisting of b hexagons arranged as in the following system. We shall also need the following Lemmas: LEMMA 3. Let S be a benzenoid system. Then *M 2 (S) = n() S h() S r() S. 3 PROOF. We have *M 2 (S) = m22 () S m23() S m33() S = n 2 () S h () S r () S r () S 9 3h () S r () S 3 = ns () hs () rs (). 3 LEMMA. Let a,b,c N so that a b +2,c 2 and let S(a,b,c) be a benzenoid system. Then n(s(a,b,c))=ac h(s) = ac + b(c) r(s)=2ac 2b(c)2c. PROOF. The first two statements are obvious. Let us prove the third one. From the following sketch Let S be an arbitrary benzenoid system and let n(s) be the number of vertices of that system, h(s) the number of hexagons of the system and r(s) the number of inlets of the system (an inlet is each path of a length of at least two on the boundary of the benzenoid system G so that its terminal vertices have degree 2 in G and its non-terminal vertices have degree 3 in G). Denote by m 22 (S) the number of connecting the vertices of degree 2, denote by m 23 (S) the number of that connect one vertex of degree 2 and one vertex of degree 3, and denote by m 33 (S) the number of that connect degree 3 vertices. This notation is taken from Rada et al. We shall use the following Lemma and Theorem as given by Rada et al. LEMMA. Let S be a benzenoid system. Then m 22 (S) = n(s)2h(s) r(s)+2 m 23 (S)=2r(S) m 33 (S)=3h(S) r(s)3. THEOREM 2. Let S be a benzenoid system. Then (S) = n() S 2 S(,2,3) 5 2 r(s). it can be easily seen that for each a,b,c N so that a b +2,c 2, we have r(s(a,b,c))=2(a) + 2(ab2)+2 (c) = 2ac 2bc 2b 2c = 2ac 2b(c)2c. THEOREM 5. Let m be any natural number. There is a set of m benzenoid systems with the same number of vertices such that each of them has the same Randi} connectivity index and no two of them have the same modified M 2 index. PROOF. Let us observe the following set of benzenoid systems F= S ( m )! ( m )!,, x: x 2, 3,..., m x x. Obviously, there are m graphs in F. For each x 2,3,...,m+, we have ( m )! ( m )! 2. x x Croat. Chem. Acta 7 (2) (2003)

3 MODIFIED ZAGREB M 2 INDEX 85 Also, for each x 2,3,...,m+, we have m m n S ( +)!, ( )!,x x x = ( m )! x =3 (m + )! x m m h S ( )! ( )!,, x x x ( m )! ( m )! x ( x ) x x r S 7 (m +)!+x ; ( m +)! m, ( )!, x x x 2 ( m )! ( m )! x 2 ( x) 2x x x Now, for each x 2,...,m+, we have 0 (m +)!2. c S ( m )! ( m )!,, x x x m m n S ( )! ( )!,, x x x 2 52 r ( m )! ( m )! S,, x x x 3( m )! 5 2 ( 0( m )! 2) 2 Thus, indeed, for two different numbers x, x 2 2,..., m+, we have c S (m+)! m, ( )!, x x x = c S (m+)! m, ( )!, x2 x2 x2 ; * M, ( m )! 2 c S (m+)!, x x x * M, ( m )! 2 c S (m+)!, x2 x2 x2 and the claim is proved. The application of Theorem 5 is demonstrated by the following example. EXAMPLE. Take m = 2. Then, F consists of the following two graphs: S(8,5,2) S(2,2,3) and *M 2 S ( m )! ( m )!,, x x x m m n S ( )!, ( )!,x x x m m h S ( )!, ( )!,x x x m r S (m+)!, ( )!,x 3 x x 3 ( m )! ( 7 ( m )! x ) ( ( m )! ) = 8 9 ( m)! x. 8 From Theorem 5, it follows that (S(8,5,2)) = (S(2,2,3)) = 3( 2 )! 5 2 (0(2+)!2). 2 On the other hand, we have *M 2 (S(8,5,2)) = ( )! ; 8 *M 2 (S(2,2,3)) = ( )! 8 so indeed (S(8,5,2)) = (S(2,2,3)) *M 2 (S(8,5,2)) *M 2 (S(2,2,3)) Croat. Chem. Acta 7 (2) (2003)

4 8 D. VUKI^EVI] AND N. TRINAJSTI] THEOREM. Let m be any natural number. There is a set F' of m benzenoid systems so that for each pair of benzenoid systems S,S 2 F' we have n(s )=n(s 2 ) M S if and only if c( S ) c( S ) * M ( S ) * ( ) PROOF. Let us consider the following set of benzenoid systems F' = Gm3, x, : x{ 2,..., m}. For each x 2,...,m+ we have n(g(m+3,x,)) = 3 (m+3). It is sufficient to prove that for each x, x 2 2,...,m+, we have (x < x 2 ) ((m+3,x,) < (m+3,x 2,)) () (x < x 2 ) (*M 2 (m+3,x,)>*m 2 (m+3,x 2,)) (5) Let us calculate for each x 2,...,m+, r(g(m+3,x,)) and h(g(m+3,x,)). h(g(m+3,x,)) = (m+3)+5 x Let us prove (5). Suppose that x,x 2 2,...m+ and x < x 2. We have *M 2 (G(m+3,x,)) = n( G( m 3, x, )) h( G( m 3, x, )) r( G( m3, x, )) ( m ) ( ( m 3 ) 5 x ) m x m x Completely analogously, we get *M 2 (G(m+3,x,)) = m x , and *M 2 (G(m+3,x,)) > *M 2 (G(m+3,x 2,)) which proves (5). The application of Theorem is demonstrated by the example given below. EXAMPLE. Take m =. Then F' consists of the following four graphs: r(g(m+3,x,)) = 2 (m+3) 2 x 52 = 2m +20x. Let us prove (). Suppose that x, x 2 2,...,m+ and x < x 2. We have S(7,2,) S(7,3,) (G(m+3,x,)) = n( G( m 3, x, )) 5 2 r( G( m 3, x, )) = 2 3( m 3) 5 2 ( 2m2 0x ) = 2 8 (m+3) (52 ) (2m+) + 5 ( 5 2 ) x. 3 Completely analogously, we get the following expression: (G(m+3,x 2,)) = 8 (m+3) (52 ) (2m+) + 5 ( 5 2 ) x2, 3 therefore which proves (). (G(m+3,x,)) < (G(m+3,x 2,)) S(7,,) S(7,5,) Croat. Chem. Acta 7 (2) (2003)

5 MODIFIED ZAGREB M 2 INDEX 87 We have (after a straightforward calculation) inequalities: (S(7,2,)) < (S(7,3,)) < (S(7,,)) <(S(7,5,)) *M 2 (S(7,2,)) > *M 2 (S(7,3,)) > *M 2 (S(7,,)) >*M 2 (S(7,5,)) as predicted by Theorem. CONCLUSIONS The modified Zagreb M 2 index was explicitly derived for benzenoid systems in terms of the number of vertices, the number of hexagons and the number of inlets, following the work by Rada et al. who derived an explicit formula for the Randi} connectivity index for benzenoids. The two main results that show that the modified Zagreb M 2 index differs, to a great extent, from the Randi} connectivity index can be summarized as: (i) for any arbitrary large m there is a number n and a set of m benzenoid systems with n vertices such that every one of them posseses the same Randi} connectivity index, but no two graphs exist with the same modified Zagreb M 2 index, and (ii) for any arbitrary large m there is a number n and a set of m benzenoid systems with n vertices so that (a) every pair of graphs possesses a different Randi} connectivity index, (b) every pair of graphs has a different modified Zagreb M 2 index, and (c) Randi} connectivity indices order these graphs reversely with respect to modified Zagreb M 2 indices. Acknowledgments. We thank the Ministry of Science and Technology of the Republic of Croatia for support. We are also grateful to the reviewers for their helpful comments. REFERENCES. N. Trinajsti}, On Modified Zagreb M 2 Index, reported at the MATH/CHEM/COMP 2002, Dubrovnik, June 229, I. Gutman and N. Trinajsti}, Chem. Phys. Lett. 7 (973) I. Gutman, B. Ru{~i}, N. Trinajsti}, and C. F. Wilcox, Jr., J. Chem. Phys. 2 (975) E. Estrada, L. Rodriguez, and A. Gutierrez, MATCH Comm. Math. Comput. Chem. 35 (997) E. Estrada, Chem. Phys. Lett. 33 (200) A. T. Balaban (Ed.), From Chemical Topology to Three-Dimensional Geometry, Plenum Press, New York, J. Devillers and A. T. Balaban (Eds.), Topological Indices and Related Descriptors in QSAR and QSPR, Gordon and Breach, Amsterdam, R. Todeschini and V. Consonni, Handbook of Molecular Descriptors, Wiley VCH, Weinheim, S. C. Basak, D. K. Harriss, and V. R. Magnuson, POLLY 2.3, copyright of the University of Minnesota, R.Todeschini, V. Consonni, and M. Pavan, DRAGON 2., M. Vedrina, S. Markovi}, M. Medi}-[ari}, and N. Trinajsti}, Comput. Chem. 2 (997) M. Randi}, J. Am. Chem. Soc. 97 (975) I. Gutman and S. J. Cyvin, Introduction to the Theory of Benzenoid Hydrocarbons, Springer-Verlag, Berlin, J. Rada, O. Araujo, and I. Gutman, Croat. Chem. Acta 7 (200) SA@ETAK Modificirani zagreba~ki M 2 indeks usporedba s Randi}evim indeksom povezanosti za benzenoidne sustave Damir Vuki~evi} i Nenad Trinajsti} Izvedena su dva teorema za benzenoidne sustave iz kojih slijedi (i) za bilo koji proizvoljno veliki broj m (npr. m= ) postoji broj n i skup s m benzenoidnih sustava i n ~vorova tako da svaki par posjeduje identi~ni Randi}ev indeks povezanosti, ali ne postoje dva benzenoidna sustava s jednakim modificiranim zagreba~kim M 2 indeksom i (ii) za bilo koji proizvoljno veliki m (npr. m = ) postoji broj n i skup s m benzenoidnih sustava i n ~vorova tako da (a) svaki par grafova posjeduje razli~iti Randi}ev indeks povezanosti, (b) svaki par grafova posjeduje razli~iti modificirani zagreba~ki M 2 indeks i (c) Randi}evi indeksi povezanosti poredaju te grafove u suprotnom redu nego to ~ine modificirani zagreba~ki M 2 indeksi. S ta dva teorema pokazano je da se modificirani zagreba~ki M 2 indeks fundamentalno razlikuje od Randi}evoga indeksa povezanosti premda su njihovi algebarski izrazi sli~ni. Croat. Chem. Acta 7 (2) (2003)